When we look at a photograph, how do we know whether it’s evidence of a historical moment, a judicious rearrangement of fact, or a total fabrication? Can we count on cameras to tell the truth or should we assume that images lie? These basic questions about the nature of the medium percolate through Faking It: Manipulated Photography Before Photoshop, a provocative study of falseness at the Metropolitan Museum of Art. Long before the advent of digital editing whittled away our faith in photographic honesty, image-makers doctored negatives, painted on prints and played around in the darkroom to advance a wide range of agendas.

Curator Mia Fineman argues that from the very beginning, photography depended on “a healthy dose of trickery”. Indeed, the assurance of authenticity virtually demanded illusion. Early technical limitations, for instance, led to bleached-out skies. Carleton Watkins solved that problem by grafting two negatives together. In his original 1867 exposure, the lofty cliffs of the Columbia River in Oregon bleed into blank white heavens. But in later prints he has corrected the problem: fluffy clouds – from a different glass plate – now feather gracefully among towering rocks. The artificial image looks more real and it also fits comfortably into the aesthetic category of the picturesque. Watkins’ 19th-century audience, weaned on the Hudson River School and attuned to the scenic allure of cumulous puffs, would understand exactly how to assimilate his souvenirs of a still exotic American continent.

Early photo-virtuosos were not about to let the limitations of the black and white spectrum quash their craving for colour. Albert Sands Southworth and Josiah Johnson Hawes applied powdery pigment to a stunning 1850 portrait of a young girl, gilding her hair, dusting her downy cheek with a pink glow, and dyeing her dress violet and pale chartreuse. Other efforts were clumsier. Susan Davis, immortalised in an unknown studio in 1865, looks positively painted. Brushstrokes glide down her dress, and the pastoral landscape that glimmers through the window looks suspiciously vivid.

Even as they tweaked their versions of reality, the first photographers touted their medium’s unique relationship to fact. William Henry Fox Talbot titled his trailblazing 1844 book The Pencil of Nature, insisting that his images were “impressed by Nature’s hand” using “optical and chemical means alone”. His fellow pioneers played to the century’s cult of scientific objectivity, advocating for the camera as the ultimate tool of disinterested observation.

The myth of the impartial lens didn’t last long. It was undermined, oddly enough, by the same desire for proof that drove the century’s pragmatists – proof, in this case, of life after death. William Mumler maintained a New York City studio where customers could sit for formal portraits with their dearly departed, who materialised as gauzy white shadows. His scam worked for a while, until someone spotted one of Mumler’s dead spirits in the flesh and living down the block. Mumler was charged with fraud and larceny in 1869, a prosecution that reversed the conventional wisdom on the photo as fact. “Who, henceforth, can trust the accuracy of a photograph?” asked one distressed newspaper columnist. “Photographs have been treasured in a belief that . . . they could not lie, but here is a revelation that they may be made to lie with a most deceiving exactness.”

And lie they did, most egregiously in Ernest Eugène Appert’s reports of “atrocities” committed by the French rebels during the Paris Commune of 1871. The pro-government Appert whipped up fury at the Communards by reconstructing scenes of massacre. He rounded up a team of models, had them re-enact brutal deeds in front of his lens, and replaced their faces with those of actual rebels. He then photographed these collages and, behold: incontrovertible proof of mob savagery. Appert’s interventions opened new horizons in doctored propaganda: Fineman shows us how the Soviets literally disappeared the people they executed, cropping them, one by one, from the history books.

The ambiguities inherent in photographic techniques have triggered alternating currents of strictness and creative licence. Near the end of the 19th century, Edward Steichen dabbled in darkroom magic, producing a memorable portrait of Rodin as shadowy apparition. He and his fellow pictorialists double-exposed, painted over, scratched out and enhanced their prints to maximise deep velvety tones and sharp contrasts. A few years later, the über-modernist Alfred Stieglitz rejected his friend’s anything-for-an-effect approach and advocated “straight photography” instead. He singled out for praise only work that was “brutally direct, devoid of flim-flam; devoid of trickery and of any ‘ism.’” Once again, the myth reigned that the camera could, and should, open a transparent window on to the world.

While the spirit of modernism soared, the tradition of manipulation went underground. But doubt has lately come back into fashion, propelled by postmodern critics who disparage the whole notion of authenticity as delusional. This is not merely an academic debate: digital tinkering has finally stripped away any lingering faith that the camera tells the unvarnished truth. As Fineman writes, “digital photography and Photoshop have taught us to think about photographic images in a different way – as potentially manipulated pictures with a strong but always mediated resemblance to things they depict.”

Well, that’s one way to think of them. But being alert to the ways of deception does not wipe out photography’s documentary powers. When Superstorm Sandy ravaged the north-east of the US, 800,000 snapshots popped up on Instagram alone, a voluminous record that was as trustworthy as it was surreal. When a few widely circulated images looked too startling to be true – such as a movie still of waves crashing against the Statue of Liberty’s knees – teams of amateur photo-fact-checkers quickly tagged them as fake. The dubious history on display in Faking It should make the casual consumer wary of being duped – and appreciative of especially artful manipulation. But it’s not all legerdemain. Sometimes even visually sophisticated people can believe what the lens has seen.